This invention relates generally to methods and apparatus for modifying the spatial exposure distribution of a laser beam. It relates also to methods and apparatus for ablating surfaces through use of a photoablative laser. Most particularly, the invention relates to methods and reprofiling for eroding the surface of the cornea through laser kerotoplasty or keratomileusis to correct certain of the vision deficiencies, such as myopia.
Lasers are commonplace today. They have wide-ranging utility in most of the applied sciences and engineering. Often, laser systems are developed to generate a laser beam with uniform cross-sectional exposure, such that any portion of the beam has approximately equal energy density. This is achieved, for example, by coupling the beam to complex optical systems, or by selecting only a small portion of the Gaussian intensity distribution, that characterizes most lasers, to approximate uniform intensity.
Alternatively, in certain uses of laser energy, it can be advantageous to employ a laser beam of predetermined and non-uniform cross-sectional intensity. Thus it is one object of this invention to provide apparatus and methods for modifying the spatial intensity distribution of a laser beam to a predetermined distribution.
In particular, lasers can be used to ablate surfaces in a controlled manner. Such ablation depends upon the interaction between the surface structure and the laser radiation, which has a known wavelength and energy density. In addition, the ablation of a surface by laser radiation is typically time-rate dependent, although this time dependency may be non-linear with ablative depth.
Improved ablative precision is often obtained through the use of pulsed laser radiation. Short pulses provide controlled depth etching in the application area. This control is especially important in photorefractive keratectomy, a type of keratomileusis procedure, where the anterior surface of the cornea is ablated to correct certain visual deficiencies such as near-sightedness. By careful application of laser radiation to the cornea, the refractive power of the eye can be changed with precision.
By successive application of laser pulses to a surface, and by altering the size of the exposure area, curvatures can be created or removed on a surface. For example, when the ablative technique begins by irradiation of a large circular area, and progressively decreases the radius of the exposed area while maintaining approximately the same energy density, the central region will have the greatest ablative depth because it is exposed to a larger amount of cumulative, ablative radiation over the duration of the procedure. Conversely, the outermost regions have the least ablative depth. If the surface were initially flat, the resulting profile would be concave. If the surface is initially convex, the surface curvature can be flattened. In photorefractive keratectomy, the curvature on the cornea, i.e. the difference between the cornea's ideal curvature and the patient's actual corneal shape, is corrected.
It is an object of this invention to provide improved apparatus and methods for performing laser kerotoplasty and keratomileusis. In the prior art, for example in U.S. Pat. No. 4,665,913, by L'Esperance entitled "Method for Ophthalmological Surgery", are disclosed for reprofiling the cornea through the use of a laser scalpel or by laser scanning techniques that are especially difficult to control. The L'Esperance '913 patent requires precise control of a laser spot on the cornea, and further requires that the complex etching pattern be carefully followed. Such scanning techniques leave much room for error and thus require expensive safeguards for surgical applications. Additionally, these methods are naturally time-consuming since only a small portion of the surface is ablated at a given time.
Accordingly, it is an object of this invention to apply laser energy to a corneal surface with improved control of using a laser that does not require scanning action.
Another technique for corneal reshaping using an laser photoablation apparatus comprises controlling the reprofiling operation by varying the size of the surface area to which the pulses of laser energy are applied. In one embodiment, a beam-shaping stop or window is moved axially along the beam to increase or decrease the area of the light incident on cornea. Alternatively, an adjustable iris can be disposed in the beam path. In either approach, by progressively altering the size of the exposed region, the desired photoablation profile is established on the corneal surface. For further details on this technique, see Marshall et al, "Photo-Ablative Reprofiling of the Cornea Using an Excimer Laser: Photorefractive Keratectomy," 1 Lasers in Ophthalmology, 21-48 (1986), and U.S. Pat. No. 4,941,093, incorporated herein by reference.
Yet another technique for corneal reshaping involves the use of a laser photoablation apparatus in which a beam-shaping mask is disposed between the laser and the surface. The mask provides a predefined profile of resistance to laser radiation erosion by selectively absorbing some of the laser radiation while permitting the remainder to be transmitted to the surface in accordance with the mask profile. For further details of such erodible masking techniques, see U.S. Pat. No. 4,856,513, 4,994,058 and 5,019,074 incorporated herein by reference.
It is another object of this invention to provide further apparatus and methods for ablating a surface, such as a cornea, by applying laser energy to the surface with selectable exposure distributions.
These and other objects of the invention are evident in the description that follows.